Four recently discovered calves and one adult cow with clinical and chemical findings characteristic of protoporphyria (EPP) constitute the first known animal model for any of the hereditary human porphyrias other than so-called "erythropoietic porphyria" (EP, congenital, or Gunther's porphyria). Of special interest was the finding of a marked uniform decrease in activity of ferrochelatase (heme synthetase) to 15 percent or less of normal in liver, heart, kidney, marrow, and other tissues studied. Heme synthetase activity in livers of four known carrier cows was one-half the normal value. Exquisite photosensitivity within a few days of birth, recessive inheritance and other findings distinguish this disease from its human counterpart. Biochemical and clinical features of this new disease are being investigated along with a number of implications related to a variety of other diseases associated with porphyrin abnormalities. Studies include the following: 1) The nature of the enzyme defect, 2) its manifestations in different tissues (porphyrin accumulation and/or hemoprotein deficiency), 3) correlation of red cell, plasma and fecal porphyrin with photosensitivity and presence of zinc complex of protoprophyrin in protoporphyria, lead poisoning, iron deficiency, sideroblastic anemia, etc., and 5) quantitative studies of differentially bound red cell prophyrin in different diseases. BIBLIOGRAPHIC REFERENCES: Schwartz, S., B. Stephenson, D. Sarkar. Quantitative estimation of urinary and other porphyrins purified by Florisil chromatography. Clin. Chem., 22:1057-1061, 1976. De Leiva, A., and S. Schwartz. The relation of pH to fluorescence of serotonin, melatonin, and other indole compounds reacted with o-phthaldialdehyde. Clin. Chem., 22:1999-2005, 1976.